Such doped semiconductor layers are for example used in organic light emitting diodes. Doping of electron transport layers (ETL) in organic light emitting diodes using alkali metals is an established technique to improve the power efficiency of such devices. Due to the unavailability of sufficiently strong molecular donors, alkali metals are used as donor atoms. Among the alkali metals, Cesium (Cs) is frequently used because it gives the highest stability of OLED with such doped ETL. Organic semiconductors are referred to as electron transport materials if whether their electron mobility is higher than their hole mobility, or their electron mobility is bigger than 1E-6 cm2/Vs (in the relevant field range of around 1-100E5V/cm). The ETL materials needs to be stable against reduction to support electron current over a long time. The energetic position of the lowest unoccupied molecular level (LUMO) is in the range from 2.4 eV to 3.8 eV, preferably 2.4 to 3 eV.
Currently, the only possibility to provide Cs vapor for doping purposes is the use of so called getter sources provided by S.A.E.S. getters. In these sources, Cs2CrO4 or Cs2MoO4 salts are decomposed by thermolysis and the released vapor guided through getter materials which removes the oxygen from the gas phase. After passing the getter, pure Cs vapor is released.
There are several drawbacks combined with this type of source. The sources need very high operating temperatures to carry out thermolysis. The container of the source achieves temperatures above 400° C. during operating, while the content is heated even more. This causes extra effort for cooling of the vacuum chamber. Due to strong gas adsorption at the getter material, severe degassing is observed during heat up, especially when the source was exposed to air beforehand. It takes a long time (ten min) until the Cs dispenser releases Cs vapor. Another ten minutes has to pass before the Cs evaporation rate is sufficiently stable. No rate is observed at quartz microbalances placed over the dispenser even when the former is water cooled. This is a major obstacle to process control. Checks for the proper operation of the source have to be carried out on a regular basis, which reduces the throughput of the vacuum chamber and adds to maintenance time and effort. Due to the complicated arrangement of the various components of such a source, it geometry flexibility is restricted, resulting in space demanding set-ups and limited evaporation characteristics. Finally, at least for Cs2CrO4, health and environmental risks are imminent due to the high toxicity of that compound.